2.1. Peripheral Neuropathies
A patient who exhibits a disorder of one or more peripheral nerves is said to suffer from a peripheral neuropathy. Peripheral nerves extend beyond the brain and spinal cord into tissues that lie outside the central nervous system to provide a bidirectional communication network. They serve as conduits of impulses from the brain and spinal cord to the rest of the body; for example, motor neurons carry signals to direct movement. Peripheral nerves are also capable of transmitting sensory information gathered by specialized receptors to the brain. In short, peripheral nerves provide the connection between brain, body, and environment, and serve to coordinate the relationship between an organism's brain and the outside world.
A peripheral neuropathy may manifest itself in a number of ways. If a motor nerve is affected, the patient may exhibit weakness in the muscle groups supplied by that nerve. If a sensory nerve is involved, the patient may experience numbness, tingling, loss of sensitivity to temperature, touch, and/or vibration, or even increased sensitivity in the area innervated by the diseased nerve.
Numerous varieties of peripheral neuropathy exist. Some are common, others are extremely rare. The etiology of certain peripheral neuropathies is well understood but some remain a mystery. Many neuropathies have been classified into particular syndromes. Each syndrome is associated with its own set of clinical symptoms and signs, prognosis, and treatment options. It is extremely important to be able to match a particular patient with the syndrome that corresponds to his or her clinical condition. Such matching, like a road map, permits the physician to choose a course of treatment and to counsel the patient as to prognosis. Often the identification of a syndrome alerts the physician to another medical condition associated with the patient's peripheral neuropathy which requires a particular course of treatment and carries its own prognosis. Accordingly, the ability to make a correct and precise diagnosis is exceedingly important in the management of a patient suffering from a peripheral neuropathy. Making the correct diagnosis may, however, be difficult. In the past, such diagnosis has depended upon an analysis of the patient's symptoms and an extremely detailed physical examination. To further complicate matters, many peripheral neuropathy syndromes have not yet been fully characterized.
Peripheral neuropathies may appear as manifestations of a wide variety of disease processes, including genetic, traumatic, metabolic, immune, and vascular disorders, as shown by Table I (see, for review, Plum and Posner, 1985, in "Pathophysiology--The Biological Principles of Disease," Smith and Thier, eds , Second Edition, W. B. Saunders Co., Philadelphia, Pa., pp. 1085-1090).
TABLE I ______________________________________ ANATOMIC CLASSIFICATION OF PERIPHERAL NEUROPATHY TWO OVERALL TYPES - 1. SYMMETRICAL GENERALIZED 2. FOCAL AND MULTIFOCAL ______________________________________ 1. Symmetrical Generalized Neuropathies (Polyneuropathies) Distal Axonopathies Toxic - many drugs, industrial and environmental chemicals Metabolic - uremia, diabetes, porphyria, endocrine Deficiency - thiamine, pyridoxine Genetic - HMSN II Malignancy associated - oat-cell carcinoma, multiple myeloma Myelinopathies Toxic - diphtheria, buckthorn Immunologic - acute inflam- matory polyneuropathy (Guillain-Barre) chronic inflammatory polyneuropathy Genetic - Refsum disease, metachromatic leukodystrophy Neuronopathies Undetermined - amyotrophic somatic motor lateral sclerosis Genetic - hereditary motor neuronopathies somatic sensory Infectious - herpes zester neuronitis Malignancy-associated - sensory neuronopathy syndrome Toxic - pyridoxine sensory neuronopathy syndrome Undetermined - subacute sensory neuronopathy syndrome autonomic Genetic - hereditary dysautonomia (HSN IV) 2. Focal (Mononeuropathy) and Multifocal (Multiple Mononeuropathy) Neuropathies Ischemia - polyarteritis, diabetes, rheumatoid arthritis Infiltration - leukemia, lymphoma, granuloma, Schwannoma, amyloid Physical injuries - severance, focal crush, compression, stretch and traction, entrapment Immunologic brachial and lumbar plexopathy ______________________________________ From Schaumburg, H., Spencer, P., and Thomas, P. K.: Disorders of Peripheral Nerves, Philadelphia, F. A. Davis Co., 1983.
Neuropathies may be classified on the basis of the anatomic component of peripheral nerve most affected. For example, some peripheral neuropathies, such as Guillain-Barre syndrome, which is associated with inflammation of peripheral nerve, is classified as a demyelinating neuropathy because it is associated with destruction of the myelin sheath that normally surrounds the nerve cell axon. In contrast, axonal neuropathies result from damage to the axon caused either by direct injury or, more commonly, from metabolic or toxic injury. In axonal neuropathy, the myelin sheaths disintegrate, as in demyelinating neuropathy, but myelin loss is secondary to deterioration of the axon. Still other neuropathies, classified as neuronopathies, are caused by degeneration of the nerve cell body; examples include amyotrophic lateral sclerosis and herpes zoster neuronitis.
Peripheral neuropathies are also classified according to the distribution of affected nerves. For example, as shown in Table I, some neuropathies are symmetrically, generally distributed, whereas others are localized to one or several areas of the body (the focal and multifocal neuropathies).
Yet another characteristic used to categorize peripheral neuropathies is the nature of the patient's symptoms, i.e., whether the patient suffers predominantly from sensory or motor abnormalities. Some peripheral neuropathies, such as amyotrophic lateral sclerosis (ALS) and the recently described Multifocal Motor Neuropathy (MMN) with conduction block are associated primarily with motor dysfunction. Others, such as paraneoplastic sensory neuropathy and neuronopathy associated with Sjogren's syndrome, are manifested by sensory abnormalities.
A brief description of several disorders of peripheral nerves as follows.